Optical polishing powder.
An example of the element Cerium

Optical polishing powder.
Cerium oxide in the form of a very fine powder is used as a polishing compound for optical glass. Supposed it works better than aluminum oxide or other abrasives in certain situations. It's very cheap: A few dollars a pound, which tells you something about how rare the "rare" earths actually are, which is not very.Source:eBay seller fellowsContributor:Theodore GrayAcquired:1 October, 2002Price: $5Size: 1"Purity: <30%Sample Group:Powders